Problem: Solve for $k$ : $13 = k - 23$
Solution: Add $23$ to both sides: $ \begin{eqnarray} \\ \hphantom{13 {+ 23}} &\hphantom{=}& \hphantom{k} \\ 13 &=& k - 23 \\ \\ {+23} && {+23} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ 13 {+ 23} &=& k \\ \end{eqnarray} $ Simplify. $ k = 36$